1. Field of the Invention
The present invention relates to a micro-strip antenna, and more particularly, to a micro-strip antenna with an L-shaped band-stop filter.
2. Description of the Prior Art
In 1953, the concept of utilizing micro-strip line antennas to transmit radio frequency signals was developed but not widely used because the micro-strip line antennas still had various defects. When a Printed Circuit Board (PCB), microwave techniques, and many kinds of low-attenuating media materials were developed, the use of micro-strip antennas became more practical. The advantages of micro-strip antennas include light-weight, small size, low cost, easy-production, and ease of attachment to any surface that is integrated with a monolithic microwave integrated circuit. In recent years, as mobile communication and personal communication became popular and well developed, micro-strip antennas have been frequently used.
In general, cellular phone size needs to be small; therefore available space for a micro-strip antenna is limited and increases design complexity. Since the design of the micro-strip antenna determines the communication quality of the cellular phone, the increased complexity for making a small micro-strip antenna has become a big challenge to the designer of the micro-strip antenna.
U.S. Pat. No. 4,180,817 provides a structure with micro-strip antennas connected in series and in parallel. However, such a structure forms a long current path, which will generate parasitic low frequency resonance. Thus, an additional band-stop filter is needed for suppressing this parasitic low frequency resonance. In U.S. Pat. Nos. 6,856,290, 7,009,564, 7,109,929, and 7,138,949, different band-stop filters are provided to improve the quality of micro-strip antennas.